1. Field of Invention
This invention relates to a solar array voltage controller system and particularly to such a solar array voltage controller involving linear control of the voltage array power bus.
2. Description of the Prior Art
A typical solar cell powered bus involves a plurality of solar cell arrays connected in parallel between a power bus and return bus (ground). In the absence of a voltage controller, the voltage level of the power bus will be at a wide range of values depending on the efficiency of the solar cells (which decreases over time), with different temperature conditions, different illumination conditions and under different load conditions. The purpose of a voltage controller system is to maintain a constant voltage or desirably near constant voltage at the power bus in spite of the above-mentioned problems which work to create varying voltage.
Various shunting structures to maintain the constant voltage are known. Some are digital, meaning that the bus voltage is maintained within a range of voltages. Some are linear, such that the bias voltage is maintained precisely but with a considerable hardware cost and weight. (In artificial satellites, where such solar cells are often employed, weight is a problem.) In one type of linear arrangement, a plurality of bipolar transistor and dissipating resistor combinations are placed in parallel between the power bus and the common bus. Excess power is dissipated in the transistors and resistors. In another parallel regulation system, the transistors are connected between tap points of respective arrays and the return bus. With such a partial shunt system, no collector dissipation resistors are involved. With the partial shunt, the various transistors are turned on and off, thus lowering the voltage across the part of each array being shunted by its respective transistor and, thus adjusting the power bus voltage to the desired value. The partial shunt is, in most respects, superior to the full shunt. A full shunt must dissipate array current at full bus potential while the partial shunt dissipates equivalent current at the lower voltage of only a portion of each array across which the transistors are placed. The disadvantage to the above described voltage controllers is in high total dissipation even though the dissipation is somewhat less with the partial shunt systems. Because of the high dissipation, large weight heat sinking structures are required.